Rigidbody means the object will be controlled by physics. If you add this component to your object then it will be totally controlled by physics engine of unity. If you want to detect collisions to your objects then you have to add rigidbody to it. By adding this component your object will come under gravitational pull. Without adding Script it will fall down due to gravity. It also enables your object to receive forces through Scripts.
You can add this component by selecting Components->Physics->Rigidbody .
Following are the properties of a rigidbody through which you can control your object:
1. Mass :- Mass of object(in kgs)
2. Drag :- you can set it either '0' which means no air resistance or infinity which means the object will stop immediately, when the objects are moving with forces.
3. Angular drag :- you can set it either '0' which means no air resistance but you can't stop the objects rotating by setting its property to infinity, when the objects rotating from torque.
4. Use Gravity :- you can enable or disable this. If it is enabled then the object of the game will be affected by gravity.
5. Is Kinematic :- It means that the object will not follow Unity's physics. It will follow the Transform or can be controlled by script.